Scanning antenna with gaseous plasma phase shifter



July 19, 1966 R. c. JONES SCANNING ANTENNA WITH GASEOUS PLASMA PHASESHIFTER Filed April 28, 1959 POWER QU T PU T F/G/ I? J /7 17 778WTAPERED LOAD 3 W r /4 l4 -l6 l MOD POWER Al 20 AMP MAG/VET 1 30 MC. W

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IN V EN TOR.

C Jon/Es Arrok/vsrs United States Patent Ofi ice 3,262,118 SCANNINGANTENNA WITH GASEOUS PLASMA PHASE SHIFTER Roger C. Jones, NorthSpringfield, Va., assignor, by mesnc assignments, to Melpar, Inc., FallsChurch, Va., a corporation of Delaware Filed Apr. 28, 1959, Ser. No.809,448 4 Claims. (Cl. 343701) The present invention relates generallyto antennas and more particularly to scanning antennas employing gaseousplasma resonant devices and cyclotron plasma resonant devices asvariable phase-shift devices which vary the directivities of theradiation patterns of the antennas.

Electrically steerable or scanning antennas are known per se. Thepresently known scanning mechanisms are, however, inherently complex andlacking in flexibility and versatility and are restricted in speed ofscan. By utilizing phase shifters to the plasma resonance types, or ofthe cyclotron plasma resonance type, in scanning antennas, it isfeasible to fabricate scanning antennas of various types, which aresimple and versatile, and in which extremely high scanning rates can beachieved.

In accordance with various embodiments of the invention, both linearscanning and circular scanning may be achieved. In the linear scannerenergy is transmitted along a wave guide or transmission line, tophase-separated radiating elements. These may be slots in the case ofWave guide, and rods in the case of transmission lines. Either the linesthemselves or containers located therein may be filled with a gaseousmedium capable of being excited to resonance, either plasma resonance orcyclotron resonance. Phase delays of different relative magnitudes, inenergy emanating from the radiating elements, may then be induced byvarying the exciting voltages or currents utilized to excite the gaseousmedium, and the magnetic field distributions employed to achievecyclotron plasma resonance.

It is, accordingly, a broad object of the present invention to provide anovel scanning antenna system, employing plasma resonance or cyclotronplasma resonance phenomena in phase shifting elements of the system.

It is a further object of the invention to provide a linear scanningantenna employing plasma resonance phase advance elements.

Another object of the invention resides in the provision of a linearscanning antenna employing cyclotron plasma resonance phase advanceelements.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIGURE 1 is a schematic representation of a linear scanning antennaarray, employing plasma resonance controllable phase shift elements;

FIGURE 2 is a schematic diagram of a modification of the system ofFIGURE 1, which employs cyclotronplasma resonance controllable phaseshift elements.

In Wide band transmission media, such as transmission lines and waveguides, the wave length of electromagnetic waves in the media is afunction of the dielectric constant of the insulating medium employed,for any fixed frequency. Effectively, this is becadse the velocity ofsuch waves is a function of dielectric constant. The relation of Wavelength in a free space, is given by A (1) xd=m where Rd is wave lengthin a medium, M is wave length in free space, and ed is dielectricconstant of the medium.

3,252,118 Patented July 19, 1.966

For wave guides and transmission lines, wave length is further affectedby line or guide constants, but for a given line or guide wave lengthcan be controlled as a function of d. i

It is pointed out in US. Patent No. 2,641,702, issued to Cohen et al.,that Ad in a wave guide or transmission line may be varied if thedielectric material is a gaseous plasma by varying electron density inthe plasma, and that this may be accomplished by varying the amplitudeof an excitation voltage. In accordance with the teachings of thispatent the excitation voltage is direct. According to the presentinvention H type excitation may be employed, i.e., excitation due toR.F. current flow in a coil. This current flow may exist at about 30 mc.for example only. In accordance with the teaching of the patent,moreover, a beam of constant direction is maintained as RF. frequency isvaried. According to the present invention, RF. frequency remainsconstant, and scanning of the beam is accomplished, as a function ofamplitude of excitation.

Referring now more particularly to FIGURE 1 of the accompanyingdrawings, the reference numeral 10 denotes a section of wave guide,which may be coupled at one end to a source of RP. energy at about 3kmc., by menas of a conventional coupling device 12. This energy travelsdown the wave guide in the TE mode to a tapered load 13, which preventsreflections. Obviously, any other mode may be utilized. In the edges ofthe wave guide 10 are located slits 14, to permit egrees of RF. energyin controlled phase relation. The distances between slits 14 areselected to provide a desired radiation pattern extending generallyperpendicular to the plane of the slits.

Located within the wave guide 10 is a glass, Pyrex or quartz tube 1-6,which extends longitudinally beyond the slits, and preferablysubstantially fills the wave guide 10 transversely, although this is byno means essential. The tube 16 includes three vertical extensions 17,about each of which is wound a few turns of wire, to form coils 18. Thecoils 18 are energized at 3 mc. from an oscillator 19, which feeds thecoils 18 via a power amplifier 20, the output of the latter beingcontrolled by a relatively low frequency modulator, say at frequenciesbelow 100,000 c.p.s.

The gas in the tube 16 may be one of He, Ar, Kr, Xe, or other inert gas,at a density of .1 to several millimeters of mercury. When excited bycurrent flowing in coils 18 a resonance plasma is formed, which has adielectric constant which is a function of degree of excitation, i.e.,of current in coils 18. By varying this current periodically, wavevelocity of RF. energy in the wave guide 10 is likewise varied, andaccordingly the phase relations of RE. energy components reaching therespective slits. Thereby, the directivity of the radiation pattern ofen ergy emitted from the slits is altered.

In one practical embodiment of the invention, a section of S-band waveguide was employed, having four slits as radiators. A gas tube 16 aboutthirteen (13) inches long was employed, the tube being fabricated of 1"\D. Pyrex glass. The glass tube was carefully baked out and evacuated,and then filled with 1.5 mm. Hg of spectroscopically pure argon gas.Four turns of copper tubing were used to excite each excitation column17, and fed in series with 600 watts of about 29 me. signal, to producethe gaseous plasma. A radiation pattern shift of about 15 was obtained,as excitation of the 29 mc. signal was varied in amplitude. Possibilityof radical improvement is indicated, on theoretical considerations, bydecreasing gas pressure, utilizing a larger active column, and betterslit design.

In accordance with a modification of the present in- 2 of theaccompanying vention, illustrated in FIGURE drawings, use of amagneto-static field, impressed across the gas tube 1 6, accompained bydecreased gas pressure, results in radically increased performance byvirtue of the principle of cyclotron-plasma resonance. The systems ofFIGURES 1 and 2 are identical except for inclusion of a magnetic fieldsource 22 in FIGURE 2 and the use of reduced gas pressure. Advantagesare (1) lower power loss, (2) less deterioration of noise figure and (3)greater band width.

The term plasma resonance indicates that the electrons in a gaseousplasma are moving in oscillatory motion, at a rate such that thereexists mechanical resonance of the electrons with the frequency of theRF. field. This term is synonymous with the term plasma oscillationswhich Tonks uses to describe the phenomena in volume 33, pages 195 ff.of the Physical Review, 1929. Plasma-cyclotron resonance, on the otherhand, implies that motion of the electrons occurs in paths affected by amagneto-static field. The latter causes the electrons to travel inspiral paths, while being oscillated in the plasma resonance mode,somewhat in the manner of ionized particles in a cyclotron.

It has been determined that at high frequencies the dielectric constantof a gaseous discharge plasma depends on the electronic charge densityin the plasma. The relation between dielectric constant and electroncharge density is I 2 e.,=e.,[1 M 1 me This frequency is precise, but infact collisions take place, which impart to the plasma a relatively wideresponse curve. The plasma in this respect acts like a singly resonantelectrical circuit having losses.

In the presence of collisions, the expression w in the above formula isreplaced by W +u In sum, the plasma acts like a singly resonant filterfor some frequency w. This frequency can be changed by changing n, theelectron density. 'If the RF. energy has a frequency adjacent to theresonant frequency, the phase of the RJF. energy suffers rapid shifts inpassing through the plasma, just as the phase of a signal passingthrough a singly resonant circuit would change were the filter slightlydetuned with respect to the frequency of the signal. Where cyclotronresonance is added, several possibilities exist. The cyclotron effect isequivalent to the addition of another tuned circuit. By proper selectionof the frequency of the latter, the effect of a doubletuned circuit maybe accomplished. Still further, by employing nonuniform excitation ofthe gas in the plasma and plasma cyclotron, a range of values of n isprovided, which has the effect of broad-banding the resonant responseband of the plasma.

While I have described and illustrated several specific embodiments ofmy invention, it will be clear that variations of the details ofconstruction which are specifically illustrated and described may beresorted to without departing from the true spirit and scope of theinvention as defined in the appended claims.

What I claim is:

1. A system for providing a scanning radiation pattern, said systemincluding a plurality of radiators, a constant frequency source of radiofrequency energy, means coupling said source of radio frequency energyto said radiators, at least one plasma resonance phase advance devicearranged to provide a phase advance of radio frequency energy of extentdetermined by amplitude of excitation of said plasma, said at least oneplasma resonance phase advance device being included in said meanscoupling said source of radio frequency energy to'said radiators, and'asource'of control energy coupled: a a r a in exciting relation to saidplasma resonance phase advance device, said source of control energybeing of such amplitude and frequency as to excite the electrons of saidplasma into resonance, and means for continuously varying the amplitudeof said control energy.

2. The combination according to claim 1 wherein said plasma resonancedevice is a cyclotron plasma resonance device.

3. An antenna array comprising plural antennas, a source of radiofrequency energy, means coupling said antennas to said source inparallel, each means including in series a plasma resonant phase advanceunit, and means for controlling the excitation of said plasma resonantphase advance units so as to vary the radiation pattern of said array,said last named means including a source of control energy applied tosaid units, said energy source being of such amplitude and frequency asto excite the electrons of said plasma into resonance.

4. A variable phase shift circuit for a source of electromagnetic energyincluding a closed tube of dielectric material containing an ionizablegas, means for coupling the energy of said source to said tube, meansexciting said gas electromagnetically at a frequency of approximatelythirty megacycles per second to excite said gas to plasma resonance, andmeans for varying the intensity of the electromagnetic excitation tovary the phase shift of the energy from said source, wherein is furtherprovided a source of magnetostatic field coupled with said gas and ofintensity sufficient to establish cyclotron plasma resonance in saidgas.

References Cited by the Examiner UNITED STATES PATENTS 2,085,406 6/1937Zwory-kin 343-909 X 2,106,770 2/ 1938 Southworth et al. 343-701 X2,466,354 4/1949 Bag-nall 343854 2,505,240 4/1950 Gorn 332--5 2,532,15711/1950 Evans 333-31 2,641,702 6/1953 Cohen et al 333-31 2,696,56612/1954 Lion et al. 30752 2,713,161 7/1955 Fiske 343 -816 2,929,0583/1960 Blasberg et al. 343--777" HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, Examiner.

W. I. SIMMONS, ELI LlEBER MAN,

Assistant Examiners.

1. A SYSTEM FOR PROVIDING A SCANNING RADIATION PATTERN, SAID SYSTEMINCLUDING A PLURALITY OF RADIATORS, A CONSTANT FREQUENCY SOURCE OF RADIOFREQUENCY ENERGY, MEANSE COUPLING SAID SOURCE OR RADIO FREQUENCY ENERGYTO SAID RADIATORS, AT LEAST ONE PLASMA RESONANCE PHASE ADVANCE DEVICEARRANGED TO PROVIDE A PHASE ADVANCE OF RADIO FREQUENCY ENERGY OF EXTENTDETERMINED BY AMPLITUDE OF EXCITATION OF SAID PLASMA, SAID AT LEAST ONEPLASMA RESONANCE PHASE ADVANCE DEVICE BEING INCLUDED IN SAID MEANSCOUPLING SAID SOURCE OF RADIO FREQUENCY ENERGY TO SAID RADIATORS, AND ASOURCE OF CONTROL ENERGY COUPLED IN EXCITING RELATION TO SAID PLASMARESONANCE PHASE ADVANCE DEVICE, SAID SOURCE OF CONTROL ENERGY BEING OFSUCH AMPLITUDE AND FREQUENCY AS TO EXCITE THE ELECTRONS OF SAID PLASMAINTO RESONANCE, AND MEANS FOR CONTINUOUSLY VARYING THE AMPLITUDE OF SAIDCONTROL ENERGY.